Magnetic resonance imaging of the functional anatomy of the inferior oblique muscle in superior oblique palsy

Postoperative magnetic resonance imaging of the inferior oblique muscle

No previous imaging study has described the appearance of the inferior oblique muscle after surgery. It is unknown whether findings signifying prior myectomy or recession are recognizable on magnetic resonance (MR) imaging and how they might differ for the two procedures. After myectomy via a temporal approach, the cauterized muscle stump retracts into the medial orbit. How far it retracts and whether it reattaches to the globe remains unclear. To address these issues, orbital MR images were reviewed in 5 patients who had previously undergone inferior oblique myectomy or recession. In each case, the operated muscle exhibited subtle but telltale features, when compared with the normal, fellow inferior oblique. After myectomy, the inferior oblique still terminated lateral to the inferior rectus muscle and appeared closely apposed to the globe, although not necessarily attached to the sclera.

Morphometry and anatomical variations of the inferior oblique muscle as relevant to the strabismus surgeries

Effective outcome of inferior oblique (IO) corrective surgeries demands a detailed knowledge of morphometry and variations of IO. Our aim was to study and morphometrically define the surgical anatomy of the IO muscle and its variations. Also to provide easily identifiable surgical coordinates to locate, the IO origin and the oculomotor nerve entry point into the IO. Dissection was performed on 16 cadaveric orbits. IO anatomy, variations, morphometry and relevant surgical distances were measured using digital caliper. IO with multiple bellies was found in five specimens. The IO mean length was 33.1 ± 3.3 mm, width at origin was 3.1 ± 0.6 mm, and width at insertion was 8.8 ± 1.5 mm. For easy localization of origin, its distance from the palpable landmarks, Zygomatico-maxillary suture and fronto-maxillary suture was measured. The mean distance between IO and the optic nerve was 10 mm. Distance of the nerve to inferior oblique entry point to the origin and insertion of the inferior oblique was measured. The nerve to IO was 28 mm long. The mean distance of the nerve entry point to IO origin was 15.5 ± 2.3 mm and distance to IO insertion was 15.2 ± 2.8 mm. A muscular bridge between the Inferior rectus (IR) & IO was found in one case, affecting ~¼ of the IO length; the distal end of the bridge was 5 mm from the IO insertion. Origin of the IO can be localized on the orbital surface of maxilla, 1-2 cm from the point where zygomatico-maxillary suture cuts the inferior orbital margin and 1-2 cm from the fronto-maxillary suture. In 19% of the orbits, the IO length was less than 30 mm, which may cause traction injury in muscle transposition procedures. The width at insertion is useful as most corrective surgeries are performed at the insertion site. The nerve to IO consistently entered at the center of medial border. The nerve entry point is important surgically as myectomy is performed between it and the insertion point. The safe distance available from the optic nerve was 7 mm. Detailed morphometry of IO may aid surgeons in better surgical planning and execution.

Can Binocular Alignment Distinguish Hypertropia in Sagging Eye Syndrome From Superior Oblique Palsy?

Purpose

Although the three-step test (3ST) is typically used to diagnose superior oblique palsy (SOP), sagging eye syndrome (SES) has clinical similarities. We sought to determine if alignment measurements can distinguish unilateral SOP from hypertropia in SES.

Methods

We studied hypertropic subjects who underwent surface-coil magnetic resonance imaging (MRI) demonstrating either SO cross-section reduction indicative of congenital or acquired palsy (SOP group) or lateral rectus muscle sag (SES group). Alignment was measured by Hess screen and prism-cover testing. Multiple supervised machine learning methods were employed to evaluate diagnostic accuracy. Rectus pulley coordinates were determined in SES cases fulfilling the 3ST.

Results

Twenty-three subjects had unilateral SOP manifested by SO atrophy. Eighteen others had normal SO size but MRI findings of SES. Maximum cross-section of the palsied SO was much smaller than contralaterally and in SES (P < 2 × 10-5). Inferior oblique cross-sections were similar in SOP and SES. In both SOP and SES, hypertropia increased in contralateral and decreased in ipsilateral gaze and was greater in ipsilateral than contralateral head tilt. In SES, nine subjects (50%) fulfilled the 3ST and had greater infraplacement of the lateral than medial rectus pulleys in the hypotropic orbit. Supervised machine learning of alignment data distinguished the diagnoses with areas under the receiver operating curves up to 0.93, representing excellent yet imperfect differential diagnosis.

Conclusions

Because the 3ST is often positive in SES, clinical alignment patterns may confound SES with unilateral SOP, particularly acquired SOP. Machine learning substantially but imperfectly improves classification accuracy.

Differences in gene expression between the primary and secondary inferior oblique overaction

BACKGROUND

This study sought to define different adaptive changes in the molecular levels of the overacting inferior oblique muscle in primary and secondary inferior oblique overaction.

METHODS

The inferior oblique muscles of patients with congenital superior oblique palsy (SOP) and those of patients with congenital esotropia were collected during surgery. RNA-seq technology was performed to detect the differentially expressed genes (DEGs) between the two groups. A comprehensive analysis of the gene expression profiles was then conducted, including the identification of DEGs, a Gene Ontology (GO) analysis, and a gene set enrichment analysis (GSEA). Finally, a protein-protein interaction (PPI) network was constructed with Search Tool for the Retrieval of Interacting Genes/Proteins (STRING) and Cytoscape software.

RESULTS

We identified 221 DEGs, of which 104 were significantly upregulated and 117 were downregulated in the SOP group. Additionally, several isoforms of the myosin heavy chain (MyHC) gene were found to be significantly and differentially expressed in the SOP group, including 3 upregulated fast-twitch MyHC isoforms (i.e., MYH1, MYH4, and MYH13) and 1 downregulated slow-twitch MyHC isoform (i.e., MYH3). The GO analysis indicated that the upregulated DEGs were mainly enriched in the muscle system process and muscle contraction. The GSEA analysis revealed that the upregulated pathways of ribosome, proteasome, oxidative phosphorylation, fatty acid metabolism, viral myocarditis, and cardiac muscle contraction were enriched.

CONCLUSIONS

Our findings provide insights into the different molecular changes of inferior oblique muscle overaction secondary to SOP and suggest the potential pathological mechanisms of inferior oblique overaction (IOOA) in SOP. The results suggest that upregulated fast-twitch MyHC isoforms and downregulated slow-twitch MyHC isoform in SOP may contribute to the increased force of its inferior oblique muscle.

Special features of superior oblique hypofunction due to tendon abnormalities

While most cases of superior oblique (SO) hypofunction represent contractile weakness due to denervation, sometimes the lesion is exclusively in the tendon. This study sought to distinguish the pattern of incomitant strabismus caused by deficiency of SO oculorotary force caused by tendon abnormalities versus that of neurogenic palsy. Clinical and magnetic resonance imaging (MRI) findings of 7 cases of unilateral SO tendon interruption or extirpation were compared with 11 cases of age matched unilateral SO palsy having intact tendons. We compared angles of misalignment with high-resolution MRI in central gaze and deorsumversion. Muscle bellies in neurogenic palsy were markedly atrophic with maximal cross sections averaging 6.5 ± 2.7 mm², in contrast with 13.5 ± 3.0 mm² contralesionally (P < .0001). In contrast, SO muscle bellies ipsilateral to tendon interruption had maximum cross sections averaging 15.1 ± 3.0 mm² occurring more posterior than on the contralesional side whose maximum averaged 12.1 ± 2.4 mm². While cross sections of SO bellies ipsilateral to tendon interruption exhibited normal contractile increase in infraduction (P < .0005), there was nevertheless strabismus with incomitance similar to that in SO atrophy. Binocular alignment was statistically similar (P > .5) in the two groups for all diagnostic positions, including head tilt, except in deorsumversion, where cases with SO tendon abnormalities averaged 20.5 ± 6.9Δ ipsilateral hypertropia, significantly more than 8.5 ± 6.6Δ in neurogenic SO atrophy (P = .001). The average difference in hypertropia Hypertropia averaged 9D greater in deorsumversion than central gaze in tendon abnormalities, but 4.1Δ less in SO atrophy (P< .019). In contralesional version, average overelevation in adduction was 1.7 (scale of 0-4) in tendon abnormalities, and 2.6 in SO atrophy (P = .23), while average underdepression in adduction was -2.3 in cases of tendon abnormalities and -1.6 in SO atrophy (P = .82). Repair of the SO tendon in three cases was effective, while alternative procedures were performed when repair was infeasible. While both denervation and tendon interruption impair SO oculorotary function, interruption causes greater hypertropia in infraversion. Surgical tightening of interrupted SO tendons may have particularly gratifying effects. Posterior SO thickening and large hypertropia in infraversion suggest SO tendon interruption that may guide a surgical strategy of tendon repair.

Functional Anatomy of Muscle Mechanisms: Compensating Vertical Heterophoria

PURPOSE

Magnetic resonance imaging (MRI) of extraocular muscle function was used to evaluate the role of newly recognized mechanisms underlying compensation of large heterophoria by vertical fusional vergence (VFV).

DESIGN

Prospective case series.

METHODS

At one academic center, 8 adults with large hyperphoria and supernormal VFV underwent MRI during monocular and binocular fixation of a centered, near target. Contractility of the rectus and superior oblique (SO) extraocular muscles in hypertropic and hypotropic eyes was determined from changes in posterior partial volume (PPV).

RESULTS

Five of 8 patients could sustain binocular fusion in the scanner. In those patients, VFV corrected approximately 5-degree misalignment, approximately 5-fold greater than normal VFV. Vertical strabismus was compensated mainly by significant contractility of the lateral more than the medial compartment of the inferior rectus (IR) in both eyes (P < .005). The superior rectus (SR) and inferior oblique muscles had no significant contractile contribution, although the hypotropic SO relaxed significantly. The IR lateral compartment and SR medial compartment significantly co-relaxed when binocular fusion was attained from monocular target fixation (P < .01).

CONCLUSIONS

Although VFV protects patients from small muscle imbalances over the lifespan, even enhanced VFV may be inadequate to avert diplopia. Compensation of hyperphoria by VFV is accomplished mainly by IR muscle relaxation in the hypotropic eye, principally in its selectively innervated lateral compartment, whereas the SO contributes little. Fusion involves compartmentally selective co-relaxation in hypotropic eye vertical rectus muscles. Taken together, these overall findings suggest a physiologic basis to prefer therapeutic surgical weakening of the medial IR in the hypotropic eye.

Quantitative analysis of structure-function relationship between ocular motility and superior oblique muscle hypoplasia in unilateral superior oblique palsy

AIMS

To determine the structure-function relationship between the degree of superior oblique (SO) hypoplasia and ocular motility in unilateral SO palsy.

METHODS

A total of 166 patients with unilateral SO palsy were divided into three groups based on their aetiology and high-resolution MRI findings by an in-plane resolution of 0.25 mm: (1) congenital SO palsy and unilateral trochlear nerve agenesis (absent group, n=79), (2) congenital SO palsy and symmetric trochlear nerves on both sides (present group, n=40) and (3) acquired SO palsy (acquired group, n=47) who all had symmetric trochlear nerves on both sides. The degree of SO hypoplasia was defined as the ratio of SO area between the paretic and nonparetic sides (SO_P/N) at the optic nerve-globe junction on MR images. Multivariate analysis was performed to investigate the relationship between SO hypoplasia and ocular motility parameters.

RESULTS

The degree of SO hypoplasia (SO_P/N) showed a weak negative correlation with bilateral head tilt differences in all groups (β=-0.009, p<0.001 in the absent group; β=-0.003, p=0.034 in the present group; β=-0.007, p=0.002 in the acquired group). There was only a weak positive correlation with SO_P/N and hypertropia differences between both gazes in the absent group (β=0.009, p<0.001) and the acquired group (β=0.007, p=0.001). In addition, none of the other ocular motility parameters were related to the degree of SO hypoplasia in all groups.

CONCLUSION

Regardless of the aetiology of unilateral SO palsy, the structure-function relationship of the paretic SO size and ocular motility examination was weak and almost negligible.

Expression of MyoD, insulin like growth factor binding protein, thioredoxin and p27 in secondarily overacting inferior oblique muscles with superior oblique palsy

Backgound

To identify and compare specific protein levels between overacting inferior oblique (IO) muscles in superior oblique (SO) palsy patients and normal IO muscles.

Methods

We obtained 20 IO muscle samples from SO palsy patients with IO overaction ≥ + 3 who underwent IO myectomies (IOOA group), and 20 IO samples from brain death donors whose IO had functioned normally, according to their ophthalmological chart review (control group). We used MyoD for identifying satellite cell activation, insulin-like growth factor binding protein 5 (IGFBP5) for IGF effects, thioredoxin for oxidative stress, and p27 for satellite cell activation or oxidative stress in both groups. Using immunohistochemistry and Western blot, we compared expression levels of the four proteins (MyoD, IGFBP5, thioredoxin, and p27).

Results

Levels of thioredoxin and p27 were decreased significantly in the IOOA group. MyoD and IGFBP5 levels showed no significant difference between the groups.

Conclusions

Based on these findings, the overacting IOs of patients with SO palsy had been under oxidative stress status versus normal IOs. Pathologically overacting extraocular muscles may have an increased risk of oxidative stress compared with normal extraocular muscles.

MRI of acquired Brown syndrome: a report of two cases

Brown syndrome is characterized by upward gaze impairment while the eye is in adduction. It is caused by abnormalities involving the superior oblique tendon-trochlea complex. Imaging can help confirm the diagnosis, shed light on its etiology, and determine the best course of treatment. However, reports of magnetic resonance imaging findings of acquired Brown syndrome are scarce in the literature. Here, we describe magnetic resonance imaging features of 2 cases of acquired Brown syndrome.

Diagnostic Utility of the Three-Step Test According to the Presence of the Trochlear Nerve in Superior Oblique Palsy

Background and Purpose

To determine the diagnostic utility of the three-step test in unilateral superior oblique palsy (SOP) according to the presence of the trochlear nerve using high-resolution thin-section magnetic resonance imaging.

Methods

In total, 166 patients with congenital and acquired unilateral SOP were included, comprising 87 with a normal trochlear nerve (present group) and 79 without a trochlear nerve (absent group). The sensitivity of each component of the three-step test was evaluated as well as factors related to the sensitivity.

Results

All three steps were positive in 78% of those in the present group and 72% of those in the absent group, demonstrating no intergroup difference (p=0.471). Superior rectus muscle (SR) contracture was more frequent in patients who exhibited at least one negative step (incomplete group) (p=0.014). The incomplete group was significantly related to larger hypertropia in ipsilateral gaze (p<0.001), smaller hypertropia in contralateral gaze (p<0.001), and smaller differences in hypertropia between ipsilateral head tilt and the primary position (p=0.012).

Conclusions

The diagnostic utility of the three-step test in unilateral SOP did not differ according to the presence of the trochlear nerve. SR contracture was the main cause of exceptions of the three-step test in unilateral SOP.

Predictive factors for corrective effect of inferior rectus recession for congenital superior oblique palsy

PURPOSE

To identify preoperative factors associated with the surgical corrective effect of contralateral inferior rectus recession (IRR) for vertical deviation in patients with congenital superior oblique palsy (SOP).

METHODS

This retrospective study included 20 treatment-naïve patients with unilateral congenital SOP (age range, 6-79 years) who underwent contralateral IRR according to our basic policy to select IRR for paretic eye fixation. The corrective effect (°/mm) of IRR was defined as the difference in the vertical deviation at the primary gaze position between before and 6-18 months after surgery per distance of recession. We also measured the preoperative vertical deviation at primary and secondary gaze positions, and vertical deviation with head-tilting, and calculated the difference in vertical deviation between these positions. We analyzed the correlation between the corrective effect of IRR and these study parameters.

RESULTS

The mean corrective effect of IRR was 2.4 ± 1.6°/mm, which had a significant correlation with preoperative differences in vertical deviation between the primary gaze position and the downward (P = 0.004, r = -0.61) and contralateral gaze positions (P = 0.03, r = -0.48); and the presence of preoperative stereopsis (P = 0.02, r = -0.51). After excluding a statistical outlier, the correlation between the corrective effect and the difference between the primary and contralateral gaze positions was no longer significant (P = 0.07), while the other two relationships remained significant.

CONCLUSIONS

Our findings suggest that preoperative differences in vertical deviation between the primary and downward gaze positions and the presence of preoperative stereopsis are important considerations prior to performing IRR for congenital SOP, particularly with paretic eye fixation.

End-organ radiographic manifestations of cranial neuropathies: A concise review

BACKGROUND

Cranial neuropathies are a spectrum of disorders associated with dysfunction of one or more of the twelve cranial nerves and the subsequent anatomic structures they innervate.

OBJECTIVE

The purpose of this article is to review radiographic imaging findings of end-organ aberrations secondary to cranial neuropathies.

METHOD

All articles related to cranial neuropathies were retrieved through the PubMed MEDLINE NCBI database from January 1, 1991 to August 31, 2014. These manuscripts were analyzed for their relation to cranial nerve end-organ disease pathogenesis and radiographic imaging.

RESULTS

The present review reveals detectable end-organ changes on CT and/or MRI for the following cranial nerves: olfactory nerve, optic nerve, oculomotor nerve, trochlear nerve, trigeminal nerve, abducens nerve, facial nerve, vestibulocochlear nerve, glossopharyngeal nerve, vagus nerve, accessory nerve, and hypoglossal nerve.

CONCLUSION

Radiographic imaging can assist in the detailed evaluation of end-organ involvement, often revealing a corresponding cranial nerve injury with high sensitivity and diagnostic accuracy. A thorough understanding of the distal manifestations of cranial nerve disease can optimize early pathologic detection as well as dictate further clinical management.

Magnetic resonance imaging of the functional anatomy of the superior oblique muscle in patients with primary superior oblique overaction

Purpose

To quantitatively determine the size and contractility of the superior oblique (SO) muscle in primary SO overaction (PSOOA).

Patients and methods

A prospective, observational study was conducted on 12 patients with PSOOA, and 10 healthy, orthotropic subjects. Sets of contiguous, 2 mm slice thickness, quasi-coronal magnetic resonance imaging were obtained during different gazes, giving pixel resolution of 0.391 mm. Cross-sectional areas of the SO muscles were determined in primary position, supraduction, and infraduction to evaluate size and contractility. The cross-sectional areas of SO muscle were compared with those of controls in the primary position to detect hypertrophy or atrophy and changes in contractility could be detected during the vertical gaze. All statistical calculations were performed using PROC MIXED (SAS 9.4).

Results

There was no difference between the ipsilesional (affected eye), contralesional (unaffected eye), and normal SO muscle cross-sections: 0.176±0.018 cm², 0.175±0.005 cm², and 0.173±0.015 cm², respectively (P=0.82). The maximum contractility of SO muscle on the ipsilesional (affected) side was 0.097±0.024 cm², and was different than on the contralesional (unaffected) side: 0.067±0.015 cm² and in control subjects: 0.063±0.018 cm² (P=0.0002).

Conclusions

In PSOOA, the ipsilesional SO is more contractile than the contralesional SO muscle and different than in controls, with no difference in SO muscle size in primary position, which suggests that excessive innervation rather than muscle hypertrophy underlies PSOOA.

Changes in Extraocular Muscle Volume During Ocular Duction

Purpose

It has been tacitly assumed that overall extraocular muscle (EOM) volume is conserved during contraction and relaxation, yet this assumption has been untested up to now. We used high-resolution magnetic resonance imaging (MRI) to determine if total EOM volume changes during relaxation and contraction.

Methods

Surface coil MRI in quasi-coronal planes was obtained in target-controlled, maximal secondary gaze positions in 30 orbits of 15 normal subjects at 312-μm resolution. Ductions were quantified by changes in globe-optic nerve positions. Cross-sections of EOM were manually outlined in contiguous image planes so that volumes could be calculated by multiplying summed cross sections by the 2-mm slice thickness. Three-dimensional reconstruction allowed measurement of the lengths of terminal, unresolvable EOM segments, providing estimates of terminal EOM volumes to be summed with measured midorbital volumes to obtain total EOM volumes.

Results

Duction range averaged 44.3 ± 4.8° from relaxation to contraction. There was a significant increase in total volume in each rectus EOM from relaxation to contraction: superior rectus (SR) 92 ± 36 mm³ (+18%, P < 10⁻¹¹); inferior rectus (IR) 51 ± 18 mm³ (+9%, P < 10⁻¹¹); medial rectus (MR) 78 ± 36 mm³ (+11%, P < 10⁻⁵); and lateral rectus (LR) 47 ± 45 mm³ (+7%, P = 0.005). Because volume changes for SR and MR exceed IR and LR, total rectus EOM volume increases in supraduction 41 ± 42 mm³ (+3.7%) and adduction 32 ± 63 mm³ (+2.3%).

Conclusions

Total EOM volume is not conserved but instead increases with contraction and decreased with relaxation. Contractile volume increases may be secondary to increased actin-myosin lattice spacing, so that density decreases. This effect is opposite that of possible hemodynamic changes.

Rectus Pulley Displacements without Abnormal Oblique Contractility Explain Strabismus in Superior Oblique Palsy

PURPOSE

Using high-resolution magnetic resonance imaging (MRI), we investigated whether rectus pulleys are significantly displaced in superior oblique (SO) palsy and whether displacements account for strabismus patterns.

DESIGN

Prospective case-control study.

PARTICIPANTS

Twenty-four patients diagnosed with SO palsy based on atrophy of the SO muscle on MRI and 19 age-matched orthotropic control subjects.

METHODS

High-resolution, surface coil MRI scans were obtained in multiple, contiguous, quasicoronal planes during monocular central gaze fixation. Pulley locations in oculocentric coordinates in the following subgroups of patients with SO palsy were compared with normal results in subgroups of patients with SO palsy: unilateral versus bilateral, congenital versus acquired, and isotropic (round) versus anisotropic (elongated) SO atrophy. Expected effects of pulley displacements were modeled using Orbit 1.8 (Eidactics, San Francisco, CA) computational simulation.

MAIN OUTCOME MEASURES

Rectus pulley positions and ocular torsion.

RESULTS

Rectus pulleys typically were displaced in SO palsy. In unilateral SO palsy, on average the medial rectus (MR) pulley was displaced 1.1 mm superiorly, the superior rectus (SR) pulley was displaced 0.8 mm temporally, and the inferior rectus (IR) pulley was displaced 0.6 mm superiorly and 0.9 mm nasally from normal. Displacements were similar in bilateral SO palsy, with the SR pulley additionally displaced 0.9 mm superiorly. However, the lateral rectus pulley was not displaced in either unilateral or bilateral SO palsy. The SR and MR pulleys were displaced in congenital SO palsy, whereas the IR and MR pulleys were displaced in acquired palsy. Pulley positions did not differ between isotropic and anisotropic palsy or between patients with cyclotropia of less than 7° versus cyclotropia of 7° or more. Simulations predicted that the observed pulley displacements alone could cause patterns of incomitant strabismus typical of SO palsy, without requiring any abnormality of SO or inferior oblique strength.

CONCLUSIONS

Rectus pulley displacements alone, without abnormal oblique muscle contractility, can create the clinical patterns of incomitant strabismus in SO palsy. This finding supports accumulating evidence that clinical binocular misalignment patterns are not reliable indicators of contractile function of the SO muscle. Ocular torsion does not correlate with and thus cannot account for pulley displacements in SO palsy.

Spontaneous Regression of Over-elevation in Adduction Following Esotropia Surgery

PURPOSE

To evaluate the course of over-elevation in adduction in patients who had esotropia following correction of horizontal deviation.

METHODS

The review of the medical files identified 28 patients who had esotropia with over-elevation in adduction. Data collected from the patients' full ophthalmological examinations included visual acuity, ocular alignment, duction, versions and sensory tests for binocularity and stereopsis, cycloplegic retinoscopy, and fundus evaluation. Oblique muscle function was graded on a scale of -4 to +4. The primary outcome measure was the amount of improvement in over-elevation in adduction following esotropia surgery.

RESULTS

Of the 28 patients (mean age: 53.5 ± 53.7 months), 13 were males and 15 were females. Of these, 22 (78.6%) had infantile esotropia and 6 (21.4%) had partial accommodative esotropia. The mean follow-up was 16.3 ± 7.9 months. All patients underwent bilateral medial rectus recession for correction of esotropia. The mean preoperative inferior oblique overaction was +2.1 ± 0.7 and the final postoperative value was +0.8 ± 0.9 (P < .001). There was a statistically significant decrease in postoperative over-elevation in adduction at all postoperative visits compared to the preoperative value (P < .001 at 3- and 6-month and final visits). Over-elevation in adduction regressed in 42 eyes (80.8%). Of these, 18 eyes (34.6%) completely resolved without any additional surgery to the inferior oblique muscle.

CONCLUSIONS

Spontaneous regression in over-elevation in adduction was observed after esotropia surgery without additional inferior oblique weakening surgery. This may affect presurgical evaluation of and surgical planning for patients.

Size of the Oblique Extraocular Muscles and Superior Oblique Muscle Contractility in Brown Syndrome

PURPOSE

This study employed magnetic resonance imaging (MRI) to investigate possible size and contractility changes in the superior oblique (SO) muscle, and possible isometric hypertrophy in the inferior oblique (IO) muscle, resulting from abnormal mechanical loading in Brown syndrome (BrS).

METHODS

High resolution orbital MRI was obtained in 4 congenital and 11 acquired cases of BrS, and compared with 44 normal subjects. Maximal cross-section areas and posterior partial volumes (PPVs) of the SO were analyzed in central gaze, supraduction, and infraduction [corrected] for the SO, and in central gaze only for the IO.

RESULTS

In congenital BrS, mean maximum SO cross-sectional areas were 24% and 20% less than normal in affected and unaffected eyes, respectively (P = 0.0002). Mean PPV in congenital BrS was also significantly subnormal bilaterally (29% and 34% less in affected and unaffected eyes, respectively, P = 0.001). However, SO muscle size and volume were normal in acquired cases. The SO muscle did not relax in supraduction in BrS, although there was normal contractile thickening in infraduction. The IO muscle had normal size bilaterally in BrS.

CONCLUSIONS

Congenital BrS may be associated with SO hypoplasia that could reflect hypoinnervation. However, unique isometric loading of oblique extraocular muscles due to restrictive hypotropia in adduction in BrS is generally not associated with changes in muscle bulk or in SO contractility. Unlike skeletal muscles, the bulk and contractility of extraocular muscles can therefore be regarded as independent of isometric exercise history. Restriction to elevation in BrS typically arises in the trochlea-tendon complex.

Superior oblique extraocular muscle shape in superior oblique palsy

PURPOSE

To investigate the superior oblique (SO) extraocular muscle cross section in normal controls and in SO palsy using high-resolution magnetic resonance imaging (MRI).

DESIGN

Prospective observational study.

METHODS

At a single academic medical center, high-resolution MRI was obtained at 312 μm in-plane resolution using surface coils in multiple, contiguous, quasi-coronal planes perpendicular to the orbital axis in 12 controls and 62 subjects with SO palsy. Previous strabismus surgery was excluded. Imaging was repeated in central gaze and infraduction. In each image plane along the SO, its cross section was outlined to compute cross-sectional area and the major and minor axes of the best-fitting ellipse. Main outcome measures were SO morphology and ocular motility.

RESULTS

The major and minor axes, cross-sectional area distributions, and volume of the SO belly were subnormal in orbits with SO palsy at most anteroposterior locations (P = .001), but discriminant analysis showed that palsied SO cross sections segregated distinctly into round and elongate shapes representing isotropic vs anisotropic atrophy, respectively. The major axis was relatively preserved in anisotropic atrophy (P = .0146). Cases with isotropic atrophy exhibited greater hypertropia in infraversion than central gaze, as well as greater excyclotorsion, than cases with anisotropic atrophy (P < .05 for all).

CONCLUSIONS

Characteristic differences in shape of the palsied SO belly correlate with different clinical features, and may reflect both the degree of differential pathology in the medial vs lateral neuromuscular SO compartments and the basis for diversity in patterns of resulting hypertropia.

Location and Gaze-Dependent Shift of Inferior Oblique Muscle Position: Anatomic Contributors to Vertical Strabismus Following Lower Lid Blepharoplasty?

PURPOSE

This study investigated, using high-resolution magnetic resonance imaging (MRI), inferior oblique muscle (IO) position relative to the adnexa in normal controls, subjects with and without vertical strabismus following lower lid blepharoplasty, and subjects with other hypertropia.

METHODS

Sagittal plane MRI was obtained in central gaze, infraduction, and supraduction in 19 controls, 11 subjects with and 2 without hypertropia following bilateral lower lid blepharoplasty, and 13 subjects with hypertropia unrelated to blepharoplasty. In the plane passing through the center of the inferior rectus muscle (IR), we analyzed IO position relative to the globe, as well as the distance from IO to the skin or orbital floor.

RESULTS

The IO was located approximately 1 mm more anteriorly and 1.2 mm more inferiorly in hypertropic than hypotropic fellow orbits of the blepharoplasty group and controls (P < 0.05). From central gaze to infraduction, IO shift in subjects with blepharoplasty was redirected inferiorly, rather than posteriorly as in all other groups. However, from central gaze to supraduction, IO motion was similar in all groups. There was scarring between the IO-IR pulley and orbital floor in the hypertropic eye after lower lid blepharoplasty.

CONCLUSIONS

Subjects with strabismus following lower lid blepharoplasty exhibit anterior and inferior IO pulley displacement in central gaze, as well as hindrance to normal posterior shift in infraduction. Proximity of IO to the orbital rim and lower eyelid skin is associated with strabismus following blepharoplasty, possibly because lower lid blepharoplasty may change lid forces on the IO-IR pulley system via scar tissue.

Magnetic resonance imaging demonstrates compartmental muscle mechanisms of human vertical fusional vergence

Vertical fusional vergence (VFV) normally compensates for slight vertical heterophorias. We employed magnetic resonance imaging to clarify extraocular muscle contributions to VFV induced by monocular two-prism diopter (1.15°) base-up prism in 14 normal adults. Fusion during prism viewing requires monocular infraduction. Scans were repeated without prism, and with prism shifted contralaterally. Contractility indicated by morphometric indexes was separately analyzed in medial and lateral vertical rectus and superior oblique (SO) putative compartments, and superior and inferior horizontal rectus extraocular muscle putative compartments, but in the whole inferior oblique (IO). Images confirmed appropriate VFV that was implemented by the inferior rectus (IR) medial compartment contracting ipsilateral and relaxing contralateral to prism. There was no significant contractility in the IR lateral compartment. The superior but not inferior lateral rectus (LR) compartment contracted significantly in the prism viewing eye, but not contralateral to prism. The IO contracted ipsilateral but not contralateral to the prism. In the infraducting eye, the SO medial compartment relaxed significantly, while the lateral compartment was unchanged; contralateral to prism, the SO lateral compartment contracted, while the medial compartment was unchanged. There was no contractility in the superior or medial rectus muscles in either eye. There was no globe retraction. We conclude that the vertical component of VFV is primarily implemented by IR medial compartment contraction. Since appropriate vertical rotation is not directly implemented, or is opposed, by associated differential LR and SO compartmental activity, and IO contraction, these actions probably implement a torsional component of VFV.

Association of superior oblique muscle volumes with the presence or absence of the trochlear nerve on high-resolution MR imaging in congenital superior oblique palsy

BACKGROUND AND PURPOSE

Congenital superior oblique palsy is known to relate to trochlear nerve absence and a variable degree of superior oblique muscle hypoplasia. The purpose of this study was to determine whether superior oblique muscle volume predicts trochlear nerve absence in congenital superior oblique palsy.

MATERIALS AND METHODS

A retrospective study of high-resolution MR imaging to evaluate the presence of the trochlear nerve and to measure superior oblique muscle areas and volumes with the image analysis tools of a PACS was performed in 128 consecutive patients with unilateral congenital superior oblique palsy and 34 age-matched healthy controls.

RESULTS

Of the 128 patients with congenital superior oblique palsy, 88 had an ipsilateral trochlear nerve absence (absent group) and 40 had both trochlear nerves (present group). In patients with congenital superior oblique palsy, the paretic side superior oblique muscle volume was significantly smaller compared with the normal side only in the absent group (P < .001). The left and right side superior oblique muscle volumes were not significantly different in controls (P = .750), and the paretic and normal side superior oblique muscle volumes were not significantly different in the present group (P = .536). The cutoff value of the paretic/normal side superior oblique muscle volume ratio for diagnosing trochlear nerve absence was ≤0.75 (sensitivity 98.9%, specificity 95.0%) in patients with congenital superior oblique palsy.

CONCLUSIONS

The ratio of paretic/normal side superior oblique muscle area and volume has an excellent predictability in diagnosing trochlear nerve absence in congenital superior oblique palsy.

Sensitivity of the three-step test in diagnosis of superior oblique palsy

PURPOSE

Although the Parks-Bielschowsky three-step test is the cornerstone of cyclovertical strabismus diagnosis, it has not been validated against an external benchmark. We evaluated the test's sensitivity in clinical diagnosis of superior oblique palsy in patients with unequivocal magnetic resonance imaging (MRI) evidence of superior oblique atrophy.

METHODS

A total of 73 strabismic patients were selected from a prospective MRI study because they exhibited superior oblique atrophy indicative of superior oblique denervation and thus confirmatory of superior oblique palsy. Of these, 50 patients who had no confounding factors were included for detailed study. Ocular motility data were evaluated to determine sensitivity of single and combined clinical findings in diagnosis of superior oblique palsy.

RESULTS

Maximum mean ipsilesional superior oblique cross section was reduced to 9.6 ± 0.6 mm(2) (mean ± standard error) in superior oblique palsy, representing 52% of the 18.5 ± 0.6 mm(2) contralesional superior oblique maximum cross section and 52% of the 18.4 ± 0.4 mm(2) control maximum superior oblique cross section (P < 0.001). Of the 50 patients, 35 (70%) with superior oblique atrophy fulfilled the entire three-step test. In 14 (28%) patients two steps were fulfilled; in 1 patient (2%), only one step. Affected superior oblique cross section was similar in orbits that fulfilled the three-step test (9.8 ± 0.9 mm(2)) and those that did not (9.1 ± 0.7 mm(2); P = 0.58).

CONCLUSIONS

The complete three-step test fails to detect 30% of cases of superior oblique atrophy. Often only two of three steps are positive in superior oblique palsy.

Magnetic resonance imaging of differential compartmental function of horizontal rectus extraocular muscles during conjugate and converged ocular adduction

Activity in horizontal rectus extraocular muscles (EOMs) was investigated by magnetic resonance imaging (MRI) of humans during asymmetric convergence to a monocularly aligned target at 15-cm distance or monocular fixation of afocal targets placed over a wide range of conjugate abduction through adduction. Cross sections and posterior partial volumes (PPVs) of EOMs were determined from quasi-coronal image planes and were separately analyzed in the inferior vs. superior compartments, defined by lines bisecting their maximum vertical dimensions. Both inferior and superior compartments of medial (MR) and lateral (LR) rectus exhibited contractile changes in PPV and maximum cross section for both asymmetric convergence and a comparable range of conjugate adduction. Both LR compartments, and the inferior MR compartment, exhibited similar decreases in contractility correlating with relaxation during both convergence and conjugate adduction. In contrast, the superior MR compartment exhibited roughly three times the contractility in conjugate adduction as in similar-magnitude convergence. In the aligned eye that did not move during convergence, summed contractility in all compartments of MR and LR exhibited corelaxation consistent with published EOM force measurements in this paradigm (Miller JM, Bockisch CJ, Pavlovski DS. J Neurophysiol 87: 2421-2433, 2002; Miller JM, Davison RC, Gamlin PD. J Neurophysiol 105: 2863-2873, 2011). The superior MR compartment also exhibited significantly greater contractility than the other compartments over the maximum achievable horizontal globe rotation from abduction to adduction. These findings suggest that the superior MR compartment is controlled differentially from the inferior compartment and suggest that its activity is reduced during convergence as a component of generally altered extraocular mechanics.

Precise size evaluation of extraocular muscles using fat-suppressed fast T1-weighted gradient-recalled echo imaging and multiple gaze fixation targets

PURPOSE

We studied the feasibility of the precise size measurement of extraocular muscles using fast magnetic resonance imaging sequences and gaze fixation targets.

MATERIALS AND METHODS

We recruited 20 healthy volunteers and optimized fat-suppressed fast T1-weighted gradient-recalled echo (FS T1-GRE) and single-shot fast spin-echo (SSFSE) imaging for evaluating extraocular muscles. With each eye at 40° abduction and adduction, we scanned the short-axis plane at the maximum diameter of the bilateral lateral and medial rectus muscles and measured the maximal cross-sectional area of the muscles during extension and contraction. We evaluated interobserver reproducibility and variability between the size measurements using the two MR sequences and the measurement reproducibility using FS T1-GRE imaging.

RESULTS

The interobserver reproducibility in the muscle size measurements using FS T1-GRE and SSFSE imaging was excellent (r = 0.93-0.94) and moderate to good (r = 0.54-0.75), respectively. The interobserver variability was less with FS T1-GRE than SSFSE imaging (p < 0.01). The reproducibility of the size measurement using FS T1-GRE was good to excellent (r = 0.78-0.92).

CONCLUSION

FS T1-GRE imaging with the subject staring at each of multiple targets is useful for evaluating precisely the size of extraocular muscles.

Function of transected or avulsed rectus muscles following recovery using an anterior orbitotomy approach

PURPOSE

To assess the function of muscles retrieved from a retrobulbar location using an anterior orbitotomy approach and to identify the prognostic factors favoring a good outcome.

METHODS

The records of all patients undergoing anterior orbitotomy for the retrieval of a transected or avulsed muscle in a retrobulbar location were reviewed. Ocular motility, before and after retrieval (with ductions scaled from -4 to +4), was evaluated.

RESULTS

Record review identified 11 patients who had suffered trauma to 12 muscles (5 inferior, 6 medial, and 1 lateral rectus muscle). Ductions improved from -4 ± 0.4 preoperatively to -2.7 ± 0.9 postoperatively (P = 0.002); mean primary position deviation improved from 34(Δ) ± 14(Δ)-15(Δ) ± 9(Δ) (P < 0.001), and mean deviation in the field of action improved from 47(Δ) ± 20(Δ)-20(Δ) ± 22(Δ) (P = 0.02). Ductions improved by at least two units in three patients, all of whom had medial rectus trauma. Single binocular vision in primary gaze was achieved in 6 patients. Patients with medial rectus muscle injury and patients injured by sinus surgery had the lowest likelihood of recovering single binocular vision.

CONCLUSIONS

Our results are similar to historical series in which muscles were not retrieved and transpositions performed; however, muscle retrieval avoids risks associated with transposition surgeries such as anterior segment ischemia. Muscle recovery via the anterior orbitotomy approach may be reasonable to consider in those cases with a reasonable possibility of having active force generation postoperatively.

Incomitant strabismus: does extraocular muscle form denote function?

The paradigm that an underacting extraocular muscle (EOM) is always atrophic or hypoplastic, and an overacting EOM should always be enlarged, leads to inconsistencies with clinical observations. It is inconsistent with the findings of normal extraocular muscle diameters in patients with apparent superior oblique muscle palsy, "overacting" inferior oblique muscles, and the superior rectus muscle overaction / contracture syndrome, among other clinical entities. These inconsistencies can be reconciled if one accepts the possibility that EOM contractile activity may reflect a change in neural input to an anatomically normal muscle, and / or that muscle contractile activity may be altered by a shift in fiber type and distribution within a normal-sized muscle. This remodeling may occur as a result of vergence adaptation or any change in neural stimulus to the muscle. There is substantial evidence to suggest that both these theoretical possibilities may likely occur.

Effects of intracranial trochlear neurectomy on the structure of the primate superior oblique muscle

PURPOSE. Although cyclovertical strabismus in humans is frequently attributed to superior oblique (SO) palsy, anatomic effects of SO denervation have not been studied. Magnetic resonance imaging (MRI) and orbital histology was used to study the effects of acute trochlear (CN4) denervation on the monkey SO. METHODS. Five juvenile macaque monkeys were perfused with formalin for 5 weeks: 15 months after unilateral or bilateral 10-mm intracranial trochlear neurectomy. Denervated and fellow orbits were imaged by MRI, embedded whole in paraffin, serially sectioned at 10-mum thickness, and stained with Masson trichrome. Whole muscle and individual fiber cross sections were quantified in SO muscles throughout the orbit and traced larger fibers in one specimen where they were present. RESULTS. MRI demonstrated marked reduction in midorbital cross section in denervated SO muscles, with anterior shift of SO mass preserving overall volume. Muscle fibers exhibited variable atrophy along their lengths. Denervated orbital layer (OL) fiber cross sections were slightly but significantly reduced from control at most anteroposterior locations, but this reduction was much more profound in global layer (GL) fibers. Intraorbital and intramuscular CN4 were uniformly fibrotic. In one animal, there were scattered clusters of markedly hypertrophic GL fibers that exhibited only sparse myomyous junctions only anteriorly. CONCLUSIONS. CN4 denervation produces predominantly SO GL atrophy with relative OL sparing. Overall midorbital SO atrophy was evident by MRI as early as 5 weeks after denervation, as denervated SO volume shifted anteriorly. Occasional GL fiber hypertrophy suggests that at least some SO fibers extend essentially the full muscle length after trochlear neurectomy.

Relationship of inferior oblique overaction to muscle bulk and position

INTRODUCTION

The possible association between inferior oblique muscle overaction and inferior oblique muscle mass has been a matter of speculation. This study was performed to evaluate the anatomical position of the inferior oblique muscle and to determine any association between muscle mass and inferior oblique muscle overaction.

METHODS

In this observational case series, eyes undergoing primary surgery for inferior oblique overaction underwent measurement of inferior oblique muscle borders distance from the limbus and muscle belly circumference.

RESULTS

Overall, 78 eyes of 51 patients with mean age of 18.3 +/- 12.3 years (range, 1-48) were evaluated. The mean distance of the anterior margin of the inferior oblique muscle from the limbus was 16.3 mm +/- 1.8 mm. The distance for the posterior margin was 20.8 mm +/- 1.9 mm. Average muscle circumference was 10.7 mm +/- 0.9 mm (range, 9 to 13). Inferior oblique muscle overaction and muscle belly circumference were not significantly correlated (p = 0.8).

CONCLUSION

There was no detectable correlation of inferior oblique overaction with either muscle position or circumference.

High-resolution magnetic resonance imaging demonstrates reduced inferior oblique muscle size in isolated inferior oblique palsy

PURPOSE

The diagnosis of isolated inferior oblique muscle palsy is controversial for 2 reasons: first, clinical findings seem inconsistent with our current understanding of oculomotor neuroanatomy and, second, similar findings can occur with other causes. Because denervated extraocular muscles atrophy, we used high-resolution magnetic resonance imaging (MRI) to assess inferior oblique muscle size in patients with clinically suspected inferior oblique muscle palsy.

METHODS

A diagnosis of inferior oblique muscle palsy in 6 patients (4 unilateral, 2 bilateral) was made clinically. High-resolution coronal and sagittal orbital MRI were obtained in subjects with clinical inferior oblique muscle palsy and in 30 age-matched control subjects. Cross sections of the inferior oblique, inferior rectus (IR), and medial rectus muscles were determined together because each is innervated by the common inferior division of the oculomotor nerve. No subject had pupillary abnormalities or other extraocular muscle weakness or restriction.

RESULTS

Mean cross-sectional area of the affected inferior oblique muscle (n = 8) at the midpoint of the inferior rectus muscle was 10.2 +/- 1.05 mm(2), which was significantly smaller than the value of 18.8 +/- 3.6 mm(2) for control subjects (n = 58, p < 0.00001). Unilaterally affected inferior oblique muscles were significantly smaller than unaffected inferior oblique muscles (p < 0.05). Mean medial rectus muscle cross section (n = 8) ipsilateral to the affected inferior oblique muscle was 36.8 +/- 2.4 mm(2), which was not significantly different from the 35.1 +/- 3.7 mm(2) value for the medial rectus muscles of control subjects (n = 61, p > 0.1). Mean inferior rectus muscle cross section (n = 8) ipsilateral to the affected inferior oblique muscle was 32.5 +/- 2.3 mm(2), which was significantly greater than the 29.9 +/- 3.3 mm(2) measurement for the control subjects (n = 61, p < 0.01).

CONCLUSIONS

We used MRI to demonstrate reduced inferior oblique muscle size in patients with clinically diagnosed inferior oblique muscle palsy, supporting the concept of isolated inferior oblique muscle weakness.

Effect of diagnostic occlusion in acquired trochlear nerve palsy

BACKGROUND

Monocular occlusion eliminates the stimulus for fusional vergence. Diagnostic occlusion may therefore be helpful in isolating the genuine profile of the fundamental ocular motility disorder, which may be an important finding regarding both differential diagnosis of strabismus and dosage of surgery. We investigated the effect of diagnostic occlusion on the motility pattern of acquired trochlear nerve palsy.

PATIENTS AND METHODS

Forty-eight patients aged between 6 and 78 years (median 49 years) with unilateral trochlear nerve palsy were first examined without patching, and then after 3 days of diagnostic occlusion. The onset of palsy was 1-35 years before (median 2 years). Squint angles localized with a dark red glass in front of the non-paretic eye were measured at a distance of 2.5 m, using the Harms tangent screen. Vertical and cyclotorsional angles in primary position (PP), 25 degrees abduction of the non-paretic eye (adduction of the paretic eye), and 25 degrees downgaze were measured.

RESULTS

The relation between hyperdeviation of the paretic eye and excyclodeviation (medians of the angles in degrees, ranges in brackets) before and after diagnostic occlusion was 5/5 and 4/6 (0;14/-1;10 and 0;19/2;13) in PP. In contralateral gaze, the relation was 8/5 and 8/6 (0;21/0;10 and 1;24/1;15), and in downgaze, 10/7 and 8/8 (0;21/1;14 and 0;23/3;18). The increase in excyclodeviation, though statistically significant (in PP, p = 0.0002) was small, with a median of 1 degree and large variability. The decrease in hyperdeviation was statistically significant in downgaze. The head-tilt phenomenon remained unchanged.

CONCLUSIONS

In patients with trochlear nerve palsy, diagnostic occlusion regularly causes an increase in excyclodeviation. In 25% of patients, this increase exceeds 3 degrees. The more variable change in vertical deviation, and the lack in change in the head-tilt phenomenon, can be explained by the fact that central gain-modulation causing an increase in both vertical deviation and the head-tilt phenomenon is not reversible within the relatively short time of 3 days. Diagnostic occlusion can eliminate compensatory innervation and may thereby release the genuine motility pattern, but the occlusion can also induce artificial squint angles.

Absence of relationship between oblique muscle size and bielschowsky head tilt phenomenon in clinically diagnosed superior oblique palsy

PURPOSE

To study whether the variation in maximum oblique muscle size accounts for individual variation in the Bielschowsky head tilt phenomenon (BHTP) in clinically diagnosed superior oblique (SO) palsy.

METHODS

Seventeen subjects with clinically diagnosed early-onset or idiopathic SO palsy and 14 normal subjects were enrolled in the study. Magnetic resonance imaging (MRI) in coronal and sagittal planes was used for quantitative morphometry of inferior oblique (IO) and SO muscles. Maximum cross-sectional area of the SO and IO cross section at the mid-inferior rectus crossing were determined in central gaze and compared with paretic eye hypertropia on ipsilesional versus contralesional head tilt.

RESULTS

Mean (+/-SD) maximum SO cross section was 18.1 +/- 3.2 mm(2) in normal subjects, 14.2 +/- 6.8 mm(2) ipsilesional to SO palsy, and 19.2 +/- 4.5 mm(2) contralesional to SO palsy. The ipsilesional SO cross section was significantly smaller than the contralesional (P = 0.004) and normal (P = 0.01) ones. The mean IO cross section was 18.3 +/- 3.5 mm(2) in normal subjects, 21.3 +/- 7.9 mm(2) ipsilesional to SO palsy (P = 0.43), and 22.0 +/- 6.7 mm(2) contralesional to SO palsy (P = 0.26). Hyperdeviation varied with head tilt by 20.1 +/- 5.5 degrees in subjects with SO atrophy, and 10.3 +/- 5.6 degrees in subjects without SO atrophy (P = 0.003). Although oblique muscle cross sections did not correlate with BHTP, subjects with clinically diagnosed SO palsy segregated into groups exhibiting normal versus atrophic SO size.

CONCLUSIONS

SO size does not account for the variation in BHTP in clinically diagnosed SO palsy, supporting the proposition that the BHTP is nonspecific for SO function.

Magnetic resonance imaging of the functional anatomy of the inferior rectus muscle in superior oblique muscle palsy

PURPOSE

Biomechanical modeling consistently indicates that superior oblique (SO) muscle weakness alone is insufficient to explain the large hypertropia often observed in SO muscle palsy. Magnetic resonance imaging (MRI) was used to investigate if any size or contractility changes in the inferior rectus (IR) muscle may contribute.

DESIGN

Prospective, case-control study.

PARTICIPANTS

Seventeen patients with unilateral SO muscle palsy and 18 orthotropic subjects.

METHODS

Surface coils were used to obtain sets of contiguous, 2-mm-thick, high-resolution, coronal MRI views in different gazes. Cross-sectional areas of the IR and SO muscles were determined in supraduction and infraduction for evaluation of size and contractility. Diagnosis of SO muscle palsy was based on clinical presentations, subnormal contractility, and SO muscle size less than the normal 95% confidence limit.

MAIN OUTCOME MEASURES

Cross-sectional areas of the IR and SO muscles.

RESULTS

Patients had 15.9+/-7.2 prism diopters (Delta; mean+/-standard deviation) of central gaze hypertropia and exhibited ipsilesional SO muscle atrophy and subnormal contractility. Mean ipsilesional, contralesional, and normal IR muscle cross-sections were 28.5+/-3.5 mm(2), 31.9+/-3.8 mm(2), and 31.8+/-5.8 mm(2), whereas mean contractility was 16.5+/-3.8 mm(2), 20.5+/-4.1 mm(2), and 16.6+/-4.8 mm(2), respectively. Ipsilesional IR muscle cross-section and contractility was significantly less than contralesional cross-section and contractility (P<0.01).

CONCLUSIONS

In SO muscle palsy, the contralesional IR muscle is larger and more contractile than the ipsilesional IR muscle, reflecting likely neurally mediated changes that augment the relatively small hypertropia resulting from SO muscle weakness alone. Recession of the hyperfunctioning contralesional IR muscle recession in SO muscle palsy is a physiologic therapy.

Inferior oblique muscle thickness in Asians

The purpose of this study was to examine the inferior oblique muscle (IOM) thickness in Asians by the quasi-sagittal plane of the orbits. Specimens were examined from 23 orbits of 18 Asians (9 orbits in males, 14 in females), aged 63 to 97 years at death. All specimens were fixed in 10% buffered formalin before use. The orbital contents were incised parallel to the optic nerve, after which the lengths of both short and long axes of the IOM were measured and the ratio of short to long axes was calculated. The mean lengths of each short or long axis of the IOM were 2.27 ± 0.49 mm and 7.23 ± 0.97 mm, respectively. The average ratio was 0.32 ± 0.07. There were no significant differences in laterality or gender. This study determined the normal IOM thickness in Asian cadavers, the outcome of which could be useful for detection of IOM involvement in Graves’ orbitopathy.

Torsion in patients with superior oblique palsies: dynamic torsion during saccades and changes in Listing's plane

BACKGROUND

The purpose was to assess intra- and post-saccadic torsion in superior oblique palsy (SOP) patients and the effect of surgery on torsion.

METHODS

Eleven patients with a presumed congenital SOP and five with acquired SOP performed 10 degrees vertical saccades over a range of +/-20 degrees. Eye movements were recorded with dual search coils. Dynamic torsion was calculated by subtracting the expected change in torsion during the saccade (based upon static torsion before and after the saccade) from the maximum intrasaccadic torsion. Eight healthy subjects were controls. We also examined the effects of surgery on dynamic torsion and the orientation of Listing's plane in patients with congenital SOP who were operated on either by weakening of the inferior oblique muscle on the affected eye (n=5), by recession of the inferior rectus muscle on the normal eye (n=4) or by both procedures (n=2). Postoperative recordings were obtained at least 1 month after surgery.

RESULTS

Patients with congenital and acquired SOP showed an increased dynamic extorsion, primarily during downward saccades. Following a recession of the inferior oblique muscle in congenital SOP patients, half showed significant decreases in extorsion (up to 1.0 degrees) during downward saccades by the affected eye. Following surgery all showed a temporal rotation of Listing's plane (up to 15 degrees for primary position).

CONCLUSION

Patients with a SOP show a characteristic pattern of dynamic torsion during vertical saccades differing from normals. Recession of the inferior oblique muscle leads to rotation of Listing's plane in all congenital SOP patients and causes large changes in dynamic torsion in a subgroup of them, perhaps reflecting the heterogeneity of congenital SOP.

Abnormalities of the oculomotor nerve in congenital fibrosis of the extraocular muscles and congenital oculomotor palsy

PURPOSE

High-resolution magnetic resonance imaging (MRI) can now directly demonstrate innervation to extraocular muscles and quantify optic nerve size. A quantitative MRI technique was developed to study the oculomotor nerve (CN3) and applied to congenital fibrosis of extraocular muscles (CFEOM) and congenital oculomotor palsy.

METHODS

The subarachnoid portions of the CN3s were imaged with a 1.5-T MRI scanner and conventional head coils, acquiring heavily T(2)-weighted oblique axial planes 1-mm thick and parallel to the optic chiasm. Thirteen normal subjects, 14 with CFEOM, and 3 with congenital CN3 palsy were included. Digital image analysis was used to measure CN3 diameter, which was correlated with motility findings.

RESULTS

In CFEOM, CN3 diameter was bilaterally subnormal in eight subjects, unilaterally subnormal in three subjects, and normal in three subjects. Mean +/- SD CN3 diameter in CFEOM was 1.14 +/- 0.61 mm, significantly smaller than the diameter in normal subjects, which measured 2.01 +/- 0.36 mm (P < 0.001). CN3 diameter variably correlated with clinical function. One subject with congenital CN3 palsy showed bilateral CN3 hypoplasia, but CN3 diameter was normal in two other subjects with congenital CN3 palsy.

CONCLUSIONS

Unilateral or bilateral hypoplasia of CN3 is quantitatively demonstrable using MRI in many cases of CFEOM and occasionally in congenital CN3 palsy. Variations in CN3 diameter in CFEOM and congenital CN3 palsy suggest mechanistic heterogeneity of these disorders that may be clarified by further imaging and genetic studies.

High-resolution magnetic resonance imaging of the extraocular muscles and nerves demonstrates various etiologies of third nerve palsy

PURPOSE

The etiology of third nerve palsy is usually diagnosed by history, motility examination, and presence of lid and pupil involvement, as well as cranial and vascular imaging. We used high-resolution magnetic resonance imaging (hrMRI) of the oculomotor nerve and affected extraocular muscles (EOMs) to investigate oculomotor palsy.

DESIGN

Prospective, noncomparative, observational case series in an academic referral setting.

METHODS

Twelve patients with nonaneurysmal oculomotor palsy of 0.75 to 252 months' duration were studied. In the orbit and along the intracranial oculomotor nerve, hrMRI at 1- to 2-mm thickness was performed. Coronal plane images of each orbit were obtained in multiple, controlled gaze positions. Structural abnormalities of the oculomotor nerve and associated changes in EOM volume and contractility were evaluated.

RESULTS

Cases were categorized as tumor related, congenital, diabetic, traumatic, and idiopathic according to clinical characteristics and hrMRI findings. Reduction of volume and contractility of affected EOMs were noted in six patients; however, there was no marked EOMs atrophy in two cases of diabetic oculomotor palsy, and there were four cases of aberrant regeneration. hrMRI demonstrated the oculomotor nerve at the midbrain and at EOMs in all cases, and in two cases with previous normal neuroimaging elsewhere that demonstrated contrast-enhancing tumors on the oculomotor nerve. One patient with apparently unilateral congenital inferior division oculomotor palsy had no detectable ipsilateral and a hypoplastic contralateral oculomotor nerve exiting the midbrain.

CONCLUSIONS

hrMRI provides valuable information in patients with oculomotor palsy, such as structural abnormalities of the orbit and oculomotor nerve, and atrophy and diminished contractility of innervated EOMs. This information could be helpful in diagnosis and management of oculomotor palsy.

Gillies Lecture: ocular motility in a time of paradigm shift

Abstract Recent progress in understanding of the structure and function of extraocular muscles, and our ability to image them clinically, allows prediction of revolutionary progress in diagnosis and treatment of strabismus in the coming decades. This perspective memorializes a lecture given in honour of Dr William Gillies, who has for decades been the paternal leader of strabismology in southern Australia.

Comparison of orbital magnetic resonance imaging in duane syndrome and abducens palsy

PURPOSE

To help resolve the clinical ambiguity between Duane syndrome with severe abduction deficit and abducens palsy, we performed orbital magnetic resonance imaging (MRI) to qualify abnormalities of the lateral rectus (LR) muscle in these entities.

DESIGN

Prospective observational case series.

METHODS

Orbital MRI was performed in 13 subjects with Duane syndrome (19 eyes), 10 subjects with chronic abducens palsy (10 eyes), and 10 orthotropic control subjects (18 eyes). High-resolution, surface coil, T(1)-weighted MRI was used to obtain contiguous, 2-mm thick quasi-coronal images of the orbits in central gaze. Digital image analysis was used to quantify cross-sectional area of the ipsilesional and contralesional LR to provide comparison with control measurements.

RESULTS

Mean maximum LR cross-sectional area in Duane syndrome was statistically similar to control (P = .454) and contralesional LR cross-sectional area (P = .227). However, in chronic abducens palsy, mean maximum ipsilesional LR cross-sectional area was markedly smaller than contralesional (P = .003) and control cross-sectional areas (P < .0001), as well as smaller than the LR in Duane syndrome (P= .0017).

CONCLUSIONS

The LR muscle in abducens palsy exhibits profound atrophy. The sparing of the LR in Duane syndrome from denervation atrophy despite absence of normal abducens innervation suggests existence of alternative LR innervation. High-resolution MRI can noninvasively demonstrate LR muscle size and distinguish Duane syndrome from chronic abducens palsy in uncertain cases.

Static Otolith-ocular Reflex Reflects Superior Oblique Muscle Disorder

PURPOSE

To elucidate the action of static otolith-ocular reflex (sOOR) in patients with superior oblique palsy (SOP).

DESIGN

Observational case series study.

METHODS

Compensatory ocular countertorsion produced by sOOR was analyzed in 12 patients with unilateral SOP and 11 normal subjects using a head-mounted measuring system.

RESULTS

When the head was tilted laterally to the ipsilateral side, the mean ratio (%) of compensatory countertorsion of the paretic eye in SOP patients to the head-tilt angle was significantly decreased compared with that in normal subjects (7 +/- 6% for patients and 17 +/- 4% for normal subjects, P < .05). Mean ratio of compensatory countertorsion of the paretic eye in nine patients with superior oblique (SO) muscle atrophy was significantly lower than that in three patients with nonatrophy on tilting to the ipsilateral shoulder (6 +/- 3% for patients with atrophy and 14 +/- 6% for patients with nonatrophy, P < .05).

CONCLUSIONS

sOOR reflects the anatomic disorder of the superior oblique muscle in SOP.

High-resolution magnetic resonance imaging demonstrates abnormalities of motor nerves and extraocular muscles in patients with neuropathic strabismus

INTRODUCTION

Although the ocular motility examination has been used traditionally in the diagnosis of strabismus that is a result of cranial nerve (CN) abnormalities, magnetic resonance imaging (MRI) now permits the direct imaging of lesions in CN palsies.

METHODS

Prospectively, nerves to extraocular muscles (EOMs) were imaged with T1 weighting in orbits of 83 orthotropic volunteers and 96 strabismic patients in quasicoronal planes using surface coils. Intraorbital resolution was 234-312 microns within 1.5- to 2.0-mm thick planes. CNs were imaged at the brainstem using head coils and T2 weighting, yielding 195 micron resolution in planes 1.0-mm thick in 6 normal volunteers and 22 patients who had oculomotor (CN3), trochlear (CN4), or abducens (CN6) palsies and Duane syndrome.

RESULTS

Oculomotor (CN3) and abducens (CN6) but not trochlear (CN4) nerves were demonstrable in the orbit and skull base in all normal subjects. Patients with congenital CN3 palsies had hypoplastic CN3s both in orbit and skull base, with hypoplasia of involved EOMs. Patients with chronic CN6 and CN4 palsies exhibited atrophy of involved EOMs. Patients with Duane syndrome exhibited absence or hypoplasia of CN6 in both orbit and brainstem regions, often with mild hypoplasia and apparent misdirection of CN3 to the lateral rectus muscle. Unlike CN6 palsy, patients with Duane syndrome exhibited no EOM hypoplasia. Patients with congenital fibrosis exhibited severe hypoplasia of CN3, moderate hypoplasia of CN6, and EOM hypoplasia, particularly severe for the superior rectus and levator muscles.

CONCLUSION

High-resolution MRI can directly demonstrate pathology of CN3 and CN6 and affected EOM atrophy in strabismus caused by CN palsies. Direct imaging of CNs and EOMs by MRI is feasible and useful in differential diagnosis of complex strabismus.

Magnetic Resonance Imaging of Human Extraocular Muscles During Static Ocular Counter-Rolling

The rectus extraocular muscle (EOM) pulleys constrain EOM paths. During visual fixation with head immobile, actively controlled pulleys are known to maintain positions causing EOM pulling directions to change by one-half the change in eye position. This pulley behavior is consistent with Listing's law (LL) of ocular torsion as observed during fixation, saccades, and pursuit. However, pulley behavior during the vestibulo-ocular reflex (VOR) has been unstudied. This experiment studied ocular counter-rolling (OCR), a static torsional VOR that violates LL but can be evoked during MRI. Tri-planar MRI was performed in 10 adult humans during central target fixation while positioned in right and left side down positions known to evoke static OCR. EOM cross-sections and paths were determined from area centroids. Paths were used to locate pulleys in three dimensions. Significant ( P < 0.025) counter-rotational repositioning of the rectus pulley arrays of both orbits was observed in the coronal plane averaging 4.1° (maximum, 8.7°) from right to left side down positions for the inferior, medial, and superior rectus pulleys. There was a trend for the lateral rectus averaging 1.4°. Torsional shift of the rectus pulley array was associated with significant contractile cross-section changes in the superior and inferior oblique muscles. Torsional rectus pulley shift during OCR, which changes pulling directions of the rectus EOMs, correlates with known insertions of the oblique EOM orbital layers on rectus pulleys. The amount of pulley reconfiguration is roughly one-half of published values of ocular torsion during static OCR, an arrangement that would cause rectus pulling directions to change by less than one-half the amount of ocular torsion.

High-resolution magnetic resonance imaging demonstrates varied anatomic abnormalities in Brown syndrome

INTRODUCTION

Although Brown syndrome classically is considered to be limited to the SO tendon sheath and trochlea, it does not always respond to SO surgery. We investigated mechanisms of Brown syndrome by magnetic resonance imaging (MRI).

METHODS

Three patients with congenital and 8 with acquired Brown syndrome were compared with matched normal subjects under a prospective protocol of high-resolution, multipositional orbital MRI using surface coils. Muscle size and contractility were determined using digital image analysis.

RESULTS

Five of 8 patients with acquired Brown syndrome had a history of trauma or surgery and demonstrated extensive scarring, avulsion, or fracture of the trochlea. One of the 8 had a cyst in the SO tendon. One congenital and one acquired case demonstrated inferior displacement of the lateral rectus (LR) pulley in adduction, with a normal SO tendon-trochlear complex. Such cases of Brown syndrome responded to surgical stabilization of the LR pulley. Two congenital cases had clinical findings of ipsilateral SO palsy confirmed on MRI by atrophy or absence of the SO belly. In congenital absence of the SO belly, the anterior tendon was present but terminated directly on the trochlea.

CONCLUSION

High-resolution MRI demonstrates a variety of abnormalities in patients presenting with Brown syndrome, including atrophy or absence of the SO belly. Management in Brown syndrome should be tailored to the pathophysiology of the individual patient.

Incomitance in monkeys with strabismus

PURPOSE

Rhesus monkeys reared with restricted visual environment during their first few months of life develop large ocular misalignment (strabismus). The purpose of this study was to describe 'A and V' patterns and DVD in these animals during fixation and eye movements and suggest that this form of rearing produces animals that are suitable model to study the mechanisms that might cause 'A/V' pattern incomitant strabismus and dissociated vertical deviation (DVD) in humans.

METHODS

Eye movements were recorded during fixation, smooth-pursuit and saccades using binocular search coils in one monkey with esotropia, three monkeys with exotropia and one normal monkey.

RESULTS

1) Monkeys reared with Alternating Monocular Occlusion or Binocular deprivation (tarsal plates intact) showed both horizontal and vertical misalignment during monocular and binocular viewing. 2) Large 'A' patterns were evident in 2 out of 3 exotropes while a 'V' pattern was observed in the esotrope. 3) Similar 'A/V' patterns were observed with either eye viewing and during fixation or eye movements. 4) The vertical misalignment, which consisted of the non-viewing eye being higher than the fixating eye, appeared to constitute a DVD.

CONCLUSION

Visual sensory deprivation methods that induce large strabismus also induce 'A/V' patterns and DVD similar to certain types of human strabismus. The source of pattern strabismus could be central, i.e., altered innervation to extraocular muscles from motor nuclei, or peripheral, i.e., altered location of extraocular muscle pulleys.